This invention relates to supercritical fluid extraction.
In supercritical fluid extraction, an extraction vessel is held at a temperature above the critical point and is supplied with fluid at a pressure above the critical pressure. Under these conditions, the fluid within the extraction vessel is a supercritical fluid. In one type of apparatus for supercritical extraction, there is a specially constructed extraction vessel within a source of heat.
A prior art apparatus for supercritical extraction of this type is described by B. W. Wright, et. al., in ANAL. CHEM. 59, 38-44 (January 1987) using a glass-lined extraction chamber within a bolted stainless steel extraction vessel heated in an oven. This type of extraction apparatus has the disadvantages of: (1) requiring time consuming steps to open the pressurized extraction vessel before use to insert the sample and again to open it after use to remove the spent sample; and (2) under some circumstances, requiring the handling of a hot extraction vessel.
Prior art apparatuses for automatically changing samples are known. For example, Beckman Instruments, Inc. has produced a radioimmuno and a biogamma analyzer that incorporates a sample changer with an elevator mechanism that raises sample vials from a sample changer to a lead-shielded radiation counting chamber above the sample chamber. Also, a gamma series 300 unit manufactured by Beckman Instruments, Inc., automatically interposes a thick lead shutter that separates the sample vial and the counting chamber from the environment outside the counting chamber. These devices are described in Beckman Bulletin 7250 dated approximately 1972 or 1973. Another apparatus was produced by Micromedic Systems, a division of Rhom and Haas, called the Micromedic Concept 4. It is described in Bulletin M1515 dated 1976.
Two patents describing systems of this type are U.S. Pat. No. 3,257,561 to Packard et al issued Jun. 21, 1966, for RADIOACTIVITY LEVEL DETECTING APPARATUS FOR SAMPLES CARRIED BY PORTABLE TRAYS WITH TRANSFER AND INDEXING MEANS FOR THE TRAYS and U.S. Pat. No. 3,198,948 to Olson issued Aug. 3, 1965, for APPARATUS FOR MEASURING ACTIVITY LEVELS OF RADIOACTIVE SAMPLES.
These devices are not suitable for handling the high temperature, high pressure fluid systems necessary for supercritical extraction.
In collecting sample (analytes) during supercritical extraction, a fluid flow restrictor is included to maintain high pressure in an extraction chamber while allowing a controlled flow rate through the sample being extracted. One type of restrictor is a length of small internal diameter tubing, often referred to as a capillary restrictor or capillary. To avoid freezing or deposition of water or other extracted substances dissolved in the fluid on the wall of the tubing, the capillary is heated. The need for heating is especially great when using a cold collection trap comprising a cold collection liquid solvent in which the outlet end of the capillary is immersed, and through which gasified extractant is bubbled.
In one prior art heated restrictor, the capillary is heated by thermal conduction along its length and by heat or enthalpy added to the fluid within the capillary, which moves along with the fluid flow to the outlet end of the capillary. The fluid discharges into a cold, dry tube of relatively large inside diameter. This larger tube then dips into the cold solvent trap. Ice and extracts build up on this tube but do not plug it because of the large diameter. This is described in international patent application number WO 92/06058, dated Apr. 14, 1992.
This arrangement is disadvantageous because it is often difficult to remove extract solidified on the inside of the large tube for assay.
It is known to directly resistance heat a member and to control the heat with a feedback system using the electrical resistance of the member to measure its temperature and compare it to a reference temperature. This technique is taught for use in a gas tube by U.S. Pat. No. 4,438,370; which is incorporated herein for reference.